METAL-INSULATOR TRANSITIONS AND PHASE-DIAGRAM OF (TI1-XVX)4O7 - ELECTRICAL, CALORIMETRIC, MAGNETIC AND EPR STUDIES

The electrical resistivity, magnetic susceptibility and EPR of (Ti 1-xVx)4O7 single crystals (0<x<1.6 at.%) have been studied between 4.2 and 300K and the heat capacity was examined in the range 120-300K. Pure Ti4O7 and the compounds with x< approximately 0.35 at.% show three phases, a low-temperature semiconducting phase with well-ordered Ti3+-Ti 3+ pairs (or bipolarons), an intermediate semiconducting phase with disordered pairs and a high-temperature metallic phase. For x>0.35 at.%, only the disordered pairs and metallic phases are found. The complete phase diagram has been obtained by electrical resistivity studies and corroborated by all other data. The electrical properties of the disordered phase are attributed to bipolaron mobility; the bipolaron binding energy Eb is estimated from these data and it is shown that vanadium induces pairs with a smaller E b in its vicinity.